PROJECT SUMMARY The long-term goal of this research is to improve our understanding of thyroid hormone action using ligand- based, small molecule approaches. Thyroid hormone has important actions related to health and disease in all tissues of the body including the central nervous system (CNS) where thyroid hormone plays a key role in regulating brain development and maintaining specialized cell populations and structures such as myelin. Compared to thyroid hormone's actions in the periphery, much less is known at the molecular and mechanistic level about its actions in the brain, and increased knowledge of thyroid hormone brain actions would be useful for developing new therapeutics for CNS disorders that intersect with thyroid hormone action including multiple sclerosis and other demyelinating diseases, and Allan-Herndon-Dudley Syndrome and other disorders involving unbalanced thyroid hormone levels in the CNS. A major limitation from a ligand/small molecule perspective is that there are currently no thyroid hormone receptor (TR) ligands, endogenous or synthetic, that distribute preferentially to the CNS from a systemically administered dose. As such, it is difficult to separate CNS from peripheral hormone effects and the lack of a therapeutic window separating desired therapeutic CNS actions from undesired systemic toxic actions is emblematic of this issue. The proposed research plan seeks to address this problem by devising and validating a pro-drug based strategy that will selectively deliver a TR ligand across the blood-brain-barrier (BBB) and into the CNS while minimizing the TR ligand exposure in circulation and peripheral organs and tissues. This selective increase in CNS vs. peripheral exposure should result in an increased window separating TR driven CNS effects from those in the periphery. The pro-drug design is based on amino-ester derivatives of TR ligands and these novel compounds will be synthesized and evaluated for CNS uptake in Specific Aim 1. CNS-selective TR activation will be assayed in Specific Aim 2 through a series of functional assays involving TR in the CNS and periphery. In Specific Aim 3, the mechanism of BBB penetration, pro-drug activation, pro-drug pharmacokinetics, and pro-drug influence on free drug in the CNS will be evaluated. This research will enable the creation of new CNS-selective TR ligands that will be useful as research tools and potentially serve as prototypes for new thyroid hormone-based therapeutics for CNS disorders.